A modulating retroreflector or MRR that can function as a dynamic optical tag includes a corner cube retroreflector or CCR, and a multiple quantum well or MQW modulator. Optical modulation using MQWs has, thus far, been the primary switching scheme used for retromodulators in a common (air-filled) CCR configuration. The primary advantage of this modulation scheme is its fast switching speeds, which can be well above 10 Mb/s, depending on the modulator size. Thus, the state of the art modulating retro-reflector has an air-filled hollow corner cube with a MQW modulator chip. For example see U.S. Pat. No. 6,154,299, by Gilbreath et al., entitled MODULATING RETROREFLECTOR USING MULTIPLE QUANTUM WELL TECHNOLOGY, issued Nov. 28, 2000; herein incorporated by reference. Because the modulating retro-reflector has a limited field-of-view, many modulating retro-reflectors are placed in a hemispherical array to cover a broader field-of-view.
MQW modulators have a limited temperature range of operation at a fixed wavelength due to the red shift of the exciton energy with temperature. This results in further degradation in the already limited (−2:1) contrast ratios available in these modulators. Moreover, the standard (air-filled) CCRs have limited field-of-view (typically <+/−30°), resulting in a limited acceptance angle for the DOT device.
Another potential optical modulation mechanism for the DOT device is through the use of optical MEMs switches. These small-size devices use their mechanical motion to change the direction or the phase of an incident optical beam. These devices are currently being extensively investigated for adaptive optics applications for which spatial light modulators using optical MEMs pixels are used to correct the distorted phase front of an optical beam.
In one implementation, the retromodulator is an optical MEMs mini corner-cube retroreflector (CCR), in which one of CCR surfaces is dynamically tilted to deflect the light beam out of the retro path, and beyond the device diffraction-limited angle, thus, resulting in amplitude modulation. The issues with using this approach are the switching speed of the modulator, currently in the 40-50 kHz range, and the small (300-400 micrometers) size of the CCRs that can result in loss of optical efficiency due to diffraction. The slow response time is due to the relatively large size of the switching corner-cube MEMs surface (300-400 micrometers), compared to more conventional optical MEMs dimensions. Reducing the corner-cube dimensions to improve the modulation speed results in further optical efficiency loss due to diffraction.
What is needed is a compact retroreflecting dynamic optical tag device with high performance characteristics, such as high data rate, high modulation contrast ratio, high field-of-view, and an extended temperature range of operation.